1. Field of the Invention
This invention relates to a photosensitive resin composition, a porous resin, a circuit board, or a wireless suspension board. More particularly, it relates to a photosensitive resin composition and a porous resin prepared therefrom which are useful to form an insulating layer of a circuit board or a wireless suspension board, and a circuit board or wireless suspension board having the porous resin as an insulating layer.
2. Description of the Related Art
Having high heat resistance, dimensional stability and insulating properties, polyimide resins have been used widely as parts or members of electrical and electronic equipment and electronic components, such as circuit boards, which are required to assure high reliability. In recent years, as the electrical and electronic equipment has been gaining in performance and function, it has been demanded to store a large quantity of information and process and transmit the information at a high speed. Accordingly, the polyimide resins for use in such fields have been required to satisfy electrical characteristics coping with higher frequencies, i.e., to have a low dielectric constant.
While the dielectric constant of plastic materials is generally decided by their molecular structure, reduction of dielectric constant of plastic materials achieved by alterations to the molecular structure is limited. As another approach, various techniques have been proposed to make plastic materials porous so that the dielectric constant may be controlled by the porosity, taking advantage of the low dielectric constant (ε) of air, which is 1.
Porous resins can be obtained by dry processes or wet processes. The dry processes include physical processes and chemical processes. The physical processes generally comprise dispersing a low-boiling liquid (blowing agent), such as chlorofluorocarbons, in a resin and heating the resin to volatilize the blowing agent thereby to generate cells to obtain foam. The chemical processes generally comprise adding a blowing agent to a resin and pyrolyzing the blowing agent to generate gas thereby to form cells and obtain foam. For example, U.S. Pat. No. 4,532,263 proposes a physical process for obtaining a polyetherimide foam structure by using methylene chloride, chloroform, trichloroethane, etc. as a blowing agent.
Processes for obtaining a foamed structure having a small cell diameter and a high cell density have recently been proposed, which comprise dissolving a gas such as nitrogen or carbon dioxide in a resin under high pressure, releasing the pressure, and heating the resin to around the glass transition point or softening point thereof to form cells. Microporous foams can be obtained by this technique. For example, JP-A-6-322168 proposes applying the technique to polyetherimide resins to obtain a heat-resistant foam. JP-A-10-45936 proposes applying the technique to styrene resins having a syndiotactic structure to obtain a foam having a cell size of 0.1 to 20 μm, which is used as a circuit member. JP-A-9-100363 discloses a low-dielectric constant insulating plastic film having a heat resistance of 100° C. or higher and a dielectric constant of 2.5 or smaller which comprises a porous resin having a porosity of 10 vol % or higher obtained by using carbon dioxide, etc. as a blowing agent.
The above-described processes of the related art of obtaining porous resins have their several disadvantages as follows. The chlorofluorocarbons used as a blowing agent in the physical processes are unfavorable for safety and the possibility of destroying the ozonophere. In addition, it is difficult with the physical processes to obtain a foamed structure with a small and uniform cell size.
The chemical processes involve the fear of the chemical blowing agent's remaining in the foam after expansion, which makes the foam unsuitable to applications where freedom from contaminants is heavily demanded, such as electrical and electronic equipment.
The techniques comprising dissolving gas in a resin under high pressure, releasing the pressure, and heating the resin to around the glass transition point or softening point to form cells have the following disadvantages. In JP-A-6-322168, the resin is impregnated with high pressure gas in a pressure vessel while being heated to or around the Vicat softening point. Therefore, when the pressure is released, the resin is in its molten state so that the high pressure gas readily expands to provide a foam which does not form fine cells. It follows that the foam, when used as a circuit board, should have a large thickness or has a limit in forming fine patterns. In JP-A-10-45936, since the glass transition temperature of the styrene-based resin is around 100° C., it is not possible to use the resin at a high temperature higher than that glass transition temperature. Furthermore, in JP-9-100363, the foam does not form so fine cells and has a limit in forming fine patterns.